So, I've been looking for a 160mm rear crankset. I know these are hard to find, and anything will be at least $100. But, I think I have a good idea that I'm going to throw out there, to see if it anyone has tried or thought of this before.
I could buy two regular 160mm right cranks. Then, take one of them and drill out the threading for the pedal. Find a counter-clockwise threaded sleeve with a lip around one end. It would be inserted into the pedal hole from behind, so the lip would keep it from being pulled through as you screwed the left pedal into it.
Get it? I can draw a picture of what I'm talking about if needed.